Chemical regeneration of acid cupric chloride etching baths utilizes schemes to re-oxidize cuprous to cupric, via for example the reactions: EQU 2CuCl+Cl.sub.2 =2CuCl.sub.2 ( 1) EQU 2CuCl+H.sub.2 O.sub.2 +2HCl=2CuCl.sub.2 +2H.sub.2 O (2)
Although such batch-processes are acceptable from the standpoint of extending the life of etching baths, they result in a net increase in solution inventory, which must eventually be disposed of. Strong environmental concerns and cost incentives exist for development of an efficient electrolytic regeneration process for these etching solutions. It is estimated that an effective electrolytic regeneration process would currently eliminate the hauling and disposal of over 20 million gallons of spent etchant per year in the United States alone.
Accordingly, several attempts have been made to develop electrolytic processes for the regeneration of acid cupric chloride etchant. These processes all suffer the drawbacks of high cost, poor efficiency and overall design complexity. In order to better understand the essential characteristics of these processes together with their inherent drawbacks, it is helpful to review the basic electrochemical principles which are pertinent to an electrolytic regeneration process. It is to be understood, however, that this discussion is not acknowledged prior art, but background for the invention only.
Thus, the overall reaction during etching of copper by cupric chloride/hydrochloric acid is: EQU Cu+CuCl.sub.2 =2CuCl (3)
The electrode reactions relevant to the etching and electrolytic regeneration processes are: ##EQU1## Cu.sup.+ ions are strongly complexed in the presence of excess chloride ions, mainly via the reaction: EQU CuCl+Cl.sup.- =CuCl.sub.2 ( 6)
A consequence of this complexation is that the thermodynamic activity of Cu.sup.+ is significantly reduced so that reaction (4) takes place at a more positive potential, while reaction (5) takes place at a correspondingly more negative potential. Depending on the degree of complexation, the relative position of the potentials of these two reactions becomes inverted so that reaction (4) occurs at a more positive potential than reaction (5). This leads to the spontaneous etching of copper, wherein reaction (4) goes from right to left and reaction (5) goes from left to right, leading to the overall reaction shown by equation (3).
The overall anodic and cathodic electrode reactions accompanying electrolytic regeneration can be written as follows: ##EQU2## The overall reaction thus comprises the anodic oxidation of Cu.sup.+ to Cu.sup.2+, and the cathodic reduction of Cu.sup.+ to Cu metal. In the absence of H.sub.2 or Cl.sub.2 evolution, as will be apparent hereinafter, the only possible source of coulombic inefficiency, in terms of the above reactions, will occur if any of the Cu.sup.+ formed by reaction (8) diffuses into the bulk of the electrolyte instead of being directly reduced to Cu metal.
Reactions (8) and (9) are consecutive electrochemical reactions. The key to successful electrolytic regeneration is that the limiting current for reaction (8) is exceeded, forcing the succeeding reaction (9) to take place. At the same time, the limiting current for reaction (7) must not be exceeded, so that the next electrochemical oxidation process, namely chlorine evolution, is avoided. Similarly, in order to avoid the occurrence of H.sub.2 evolution at the cathode, the limiting current for the combined reactions (8) and (9) must not be exceeded.
The standard potentials for the hydrogen and chlorine evolution reactions are more negative and more positive, respectively, than either the cupric or cuprous ion reactions above, namely, ##EQU3## The occurrence of these reactions can in principle be avoided by careful control of solution mass transfer and current density as will be hereinafter established.
Moreover, in a commercial spray etcher, depending on fluid tightness of the spray etcher design, it is inevitable that there will be some degree of oxygen ingress which, if unchecked, will gradually consume both CuCl and HCl according to reaction (14), EQU O.sub.2 +4HCl+4CuCl=4CuCl.sub.2 +2H.sub.2 O (14)
This reaction will have the effect of driving the redox potential positive. It also leads to a net growth in solution inventory, analogously to the chemical regeneration schemes which electrolytic regeneration seeks to avoid. Although this effect will be alleviated to some extent by drag-out, this is not sufficient in a well designed spray etcher system. Furthermore, it is important to avoid such losses in order to minimize the need for off-line waste treatment. The magnitude of oxygen ingress can be as much as 5% of the copper throughput at the spray etcher, and there will thus be an overall current efficiency penalty of the same magnitude.
In what is believed to have been the first effort to develop a commercial electrolytic system for regeneration of an acid cupric chloride etching bath, Parikh and coworkers [Metal Finishing, p. 42, March 1972; U.S. Pat. No. 3,784,455], following an earlier patent by Garn and Sharpe [U.S. Pat. No. 2,964,453], devised an arrangement in which the cathode consisted of a bundle of cylindrical rods, and the anode was planar graphite. The (immersed) anode to cathode area ratio was between five and six to one. The cathode rods were arranged so that they could be sequenced into and out of the electrolyte. While the rods were external to the electrolyte, electrodeposited copper could be scraped off.
More recently, a system has been proposed by R. Ott and H. Reith [U.S. Pat. No. 4,508,599] wherein the cathode is configured as a narrow rotating wheel, having an outer titanium band, the sides of the wheel being of nonconductive plastic. The cathode wheel is rotated slowly within a semicircular vessel having a coating of mixed transition metal oxides as catalyst for the anodic oxidation reaction. The relative areas of the anode and cathode were set at 7.5:1. Copper metal was scraped off the cathode during the period that the titanium surface was external to the electrolyte. In each of the examples described above, the anolyte and catholyte are common to each other.
Another system described by Hillis [Soc. Chem. Ind., p. 91 (1980); U.S. Pat. No. 4,468,305] employs separate anolyte and catholyte flow loops having different electrolyte compositions, the catholyte being approximately 10 times more dilute in copper than the anolyte. In this way, the need for different anode and cathode areas is avoided since the limiting current for Cu.sup.2+ going to Cu.sup.+ is exceeded simply by virtue of the electrolyte concentration difference. Electrode separation is provided in this case by a cation exchange membrane. Electrodeposited copper was removed from the cathodes by means of a scraping mechanism.
The above-described processes and apparatus each have serious drawbacks, including having different anode and cathode areas suffering the problem of uneven current distribution. The latter translates directly into uneven potential distribution, particularly at the anode, which can render it impossible to avoid parasitic Cl.sub.2 evolution at those regions farthest removed from the cathode. Waste heat generation stemming from iR losses is excessive in each case, as a direct consequence of the high operating voltages, namely 5 to 8 volts. This results in the need for costly heat exchangers in addition to being an additional operating cost burden. The anodes employed in each of the examples noted above are all flow-by electrodes which are inherently inefficient and restrict freedom in selection of design parameters.